1. Field of the Invention
The present invention relates to blasting with explosives. More particularly, a blasting method is provided whereby a clean, substantially smooth and flat vertical rock face can be achieved in the excavation of highway and railroad cuts, canals, quarries, mines, building sites and the like in rock.
In the excavation of rock from a work site, it is often important that the remaining wall of, for example, a cutting or quarry face be left as clean and smooth as possible. Such clean walls decrease the amount of loose material on the sides resulting in greater safety and a reduction in the labour required to clear any such loose material. In mining operations, a relatively smooth wall results in greater stability of pit and backwalls. This allows steeper slope angles, thereby reducing the overall cost of mining an ore body. Additionally, where excavation walls are to be faced with concrete, the presence of a smooth and regular wall face reduces the amount of concrete needed with consequent savings in both labour and materials.
2. Description of the Prior Art
Heretofore, smooth wall blasting has been undertaken by employing several different methods variously known as perimeter blasting, pre-shearing, pre-splitting, contour blasting, cushion blasting and buffer blasting.
In one method, very small diameter holes have been drilled vertically into the rock in close spacing. These holes are, then, charged with explosives which, upon detonation, produce a clean crack or shear in the rock extending between each hole and reaching from the mouth to the base of each hole. Subsequent excavation of the rock leaves a smooth, flat wall. This method is costly since a very large number of small diameter holes must be drilled and expensive, sensitive, small diameter explosive cartridges are required.
A second method employing the same principle requires the drilling of aligned, medium diameter boreholes at spaced intervals in the rock for explosive charging. One or more intermediate unfilled boreholes are drilled between the charged boreholes. When the charged boreholes are detonated, the rock tends to be sheared along the entire line of boreholes. This method, commonly referred to as pre-shearing, requires the drilling of a substantially large number of boreholes in order to achieve a smooth wall result. In harder rock, holes may be required to be spaced every meter or less in order to produce the desired results.
Another technique of producing substantially reduced blast-induced damage to backwall involves reducing the amount of explosives in the last row of holes in a multiple row blast. These holes, sometimes drilled at an angle towards the pit floor, contain a full explosive load at the bottom but contain decked or smaller diameter, decoupled charges along the column. This last row of holes can be fired ahead or along with the main production blast. This technique is known as buffer blasting.
Of the above techniques, only pre-shearing employing small diameter holes and small spacings yields acceptable smooth wall results. The other techniques merely reduce the degree of overbreak along the backwall, compared to regular production blasts.
In a more recently developed technique, the boreholes may be reduced in number by means of notching whereby the inner walls of the boreholes are scored or notched at their circumference along the line of desired shear. Such borehole notching requires the use of special drilling equipment and techniques and has not proven to be particularly cost effective or successful.